The present invention relates to a sheet feeding device that conveys a transfer sheet for image forming, and to an image forming apparatus equipped with the sheet feeding device.
In an ordinary image forming apparatus, a transfer sheet stacked in a sheet feed cassette is fed out by a sheet feed roller, then, a separation roller separates a single transfer sheet which is conveyed to a registration roller along a sheet feed path. On the other hand, in the vicinity of a rotating image carrier, there are arranged a charging means, an image-wise exposure means and a developing means, and a photosensitive surface of the image carrier is electrically charged uniformly, and then is subjected to image-wise exposure, thus, there is formed a latent image that is developed to turn into a toner image. A registration roller that stops rotating and rotates for conveyance synchronizes with the toner image on the carrier to convey a transfer sheet, and thereby the toner image is transferred by a transfer means onto the transfer sheet in a transfer section. The transfer sheet onto which the toner image has been transferred is subjected to fixing in a fixing unit where the toner image is fixed, and then, is ejected out.
Even in the case of the image forming apparatus stated above, a high speed printer having high capacity for processing is requested. Conditions of the high speed printer are that (1) a peripheral speed of the rotating image carrier is high, and a conveyance speed for a transfer sheet is high and (2) intervals of conveyed and ejected transfer sheets are small. High speed printers satisfying the conditions of (1) and (2) are now studied actively.
When conducting continuous print, a registration roller repeats a cycle of rotation/stop/rotation, and in the course of rotation, there is conducted conveyance of a transfer sheet to a transfer area. When the registration roller is in the state of stop, it corresponds to the intervals of transfer sheets in the aforesaid condition (2). A leading edge of the succeeding transfer sheet needs to reach the position of the registration roller within the period of suspension of the registration roller, and if the leading edge of the succeeding transfer sheet reaches the position of the registration roller after the registration roller has started its rotation, the interval of the conveyed transfer sheet is made longer, resulting in fluctuation of sheet intervals and slippage of transferred toner images. These are caused by fluctuation of sheet feed timing.
As a technology to reduce fluctuations of sheet intervals in the past, there has been taken a method to provide a transfer sheet detection means in a sheet feed path, and thereby to detect the trailing edge of the preceding sheet and to conduct preliminary feeding of the succeeding sheet based on the detection mentioned above. In this case, when a jam occurs or a job is finished, the transfer sheet that has been fed preliminarily is suspended between a sheet feed tray and a main body. Therefore, a mechanism for returning the transfer sheet back to the sheet feed tray is needed.
Further, to secure sheet intervals for transfer sheets, a transfer sheet detection means is provided in a sheet feed path, and thereby, the trailing edge of the preceding transfer sheet is detected and sheet feed timing for the succeeding transfer sheet is changed for control. In this case, when sheet feed timing is delayed, it causes a fall of processing capacity, which is a drawback that the processing capacity is not constant.
For the purpose of setting the sheet intervals for transfer sheets, it is necessary to make fluctuation of sheet feeding timing small. Inventors of the invention paid attention to that the fluctuation of sheet feed timing was caused in a process including a sheet feed cassette up to sheet feeding and sheet separation, and have achieved a technology to reduce the fluctuation of sheet feed timing between a separation roller to a registration roller.
A first object of the invention is to provide a sheet feeding device in which the fluctuation of sheet feed timing is reduced in a sheet feed path up to a registration roller. A second object of the invention is to provide an image forming apparatus in which the sheet feeding device mentioned above is equipped and high speed printing is conducted.
The first object of the invention is attained by a sheet feeding device wherein a detection means that detects sheet running between a separation roller that separates a transfer sheet for conveyance and a registration roller that feeds a transfer sheet in synchronization with an image formed on an image carrier, and thereby, the detection means judges whether the state of conveyance of the transfer sheet conveyed through the separation roller is in a quick range or not, and sheet feed control at the upstream side of the registration roller is made based on the results of the judgment.
Further, the first object is attained by a sheet feeding device having the following structures: a conveyance path along which a recording sheet is conveyed; a separation roller that separates a recording sheet and conveys it; a registration roller that stops temporarily a recording sheet separated and conveyed by the separation roller and further conveyed along the conveyance path, and conveys the recording sheet again toward the image forming position at the prescribed timing; a detector that is provided in the conveyance path between the separation roller and the registration roller, and detects the recording sheet; a recognizing means that recognizes fluctuation of conveyance timing of the recording sheet based on the results of the detection made by the detector; and a control means that conducts conveyance control for the recording sheet at the upstream side of the registration roller based on the results of the recognition made by the recognizing means.
The second object of the invention is attained by an image forming apparatus forming a toner image on an image carrier and transferring the toner image onto a transfer sheet that is separated for conveyance by a separation roller and conveyed by a registration roller in synchronization, wherein, a detection means that detects sheet running is provided in a sheet feed path between the separation roller and the registration roller, and thereby, the detection means judges whether the state of conveyance of the transfer sheet conveyed through the separation roller is in a quick range or not, and sheet feed control at the upstream side of the registration roller is made based on the results of the judgment.
Further, the second object is attained by an image forming apparatus having the following structures: a sheet containing section that contains recording sheets; a sheet feed roller for feeding the recording sheet from the sheet containing section; a separation roller that separates the recording sheet fed by the sheet feed roller for conveyance; a conveyance path along which the recording sheet separated for conveyance by the separation roller is conveyed; a registration roller that stops temporarily a recording sheet conveyed along the conveyance path, and conveys the recording sheet again toward the image forming position at the prescribed timing; a detector that is provided in the conveyance path between the separation roller and the registration roller, and detects the recording sheet; a recognizing means that recognizes fluctuation of conveyance timing of the recording sheet based on the results of the detection made by the detector; and a control means that conducts conveyance control for the recording sheet at the upstream side of the registration roller based on the results of the recognition made by the recognizing means.